In a large bowl, whisk together milk and eggs until combined. Add ground beef, Italian sausage, breadcrumbs, Parmesan cheese, salt, pepper and garlic powder; stir with a wooden spoon or mix with your fingers until well combined.
Heat broiler to high; place an oven rack about 4 to 5 inches from the broiler. Lightly grease 2 baking sheets with cooking spray or oil. Using your hands, shape (about 40) 1 to 2-inch meatballs from the mixture; place on prepared baking sheets.
Broil meatballs 6 to 8 minutes, flipping halfway through, until browned. Transfer to a 6-quart slow cooker.
Top meatballs evenly with sauce. Cover and cook on low 4 to 5 hours in the slow cooker until meatballs are cooked through to an internal temperature of at least 160 degrees F.
Serve on top of pasta, on split hoagie rolls with melty cheese for meatball sub sandwiches, alongside buttery mashed potatoes or on their own as an appetizer.
TO FREEZE: I froze mine in the sauce after everything was cooked in the slow cooker with great success, but you could also freeze the broiled/browned meatballs out of the sauce. Just be sure to thaw them in the fridge overnight before cooking them in the slow cooker with the sauce.
NOTE: This recipe makes a lot of meatballs, so if you don’t plan to freeze any I’d suggest halving the recipe.